Your problem sounds similar to one I’m just recovering from although you really need to see a professional for a proper diagnosis.
Like you, I woke one morning with a small pain by my shoulder blade. I ignored it, thinking I had ‘slept funny’ on it that night. Over the next month it got worse and worse until my back, shoulder, neck and arm were excruciating painful at times, so much so that I was on the verge of going to A&E for some pain relief, as OTC meds weren’t touching it.
I was struggling to get an appointment at my surgery so in the end, I paid to see a private physiotherapist, who diagnosed a problem with the trapezius muscle in my back - a body part I had no idea I possessed! She worked on the different areas and used ultrasound, telling me it would probably get worse before it got better. She wasn’t wrong, but 48hrs later and doing the exercises she gave me, I finally began to feel some relief.
I also managed eventually to see someone at my surgery, who diagnosed the same problem. I am 75% recovered, no longer taking pain killers and hope to get out into the garden again next week to tidy up and plant bulbs. It made me feel old, that’s for sure, so I sympathise with you.